Update OpenAPI tags

[CasperWA]

Fixes #120

Remove entry endpoints from all /info endpoints.
Make all tags plural.

[codecov]

Codecov Report

Merging #121 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #121   +/-   ##
=======================================
  Coverage   82.81%   82.81%           
=======================================
  Files          39       39           
  Lines        1810     1810           
=======================================
  Hits         1499     1499           
  Misses        311      311

Flag	Coverage Δ
#unittests	82.81% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
optimade/server/routers/references.py	100% <ø> (ø)	⬆️
optimade/server/routers/info.py	95.83% <ø> (ø)	⬆️
optimade/server/routers/structures.py	100% <ø> (ø)	⬆️

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[ltalirz]

thanks @CasperWA , I agree that this reduces confusion